The Heritage Conservancy preserves the Durham bat mine. But Yerger says the bat population here is being decimated.

"It runs anywhere from a 95-99% mortality," she said.

After being hit with a fungus called White Nose Syndrome, there are only a few hundred bats left in the Bucks County hibernacula.

The disease was first identified in New York in 2007, and it irritates the bats' muzzle, ears and wings.

"If they wake up repeatedly it burns their fat stores, and with that they will literally starve to death halfway through the winter," said Yerger.

White Nose Syndrome is killing bat colonies across the Northeast at an alarming rate.

"It has affected anywhere from 5 to 6-million bats," added Yerger. "And you have to understand that bats were our most plentiful mammal in North America and that may no longer be the case."

Experts say if bats continue to die in record numbers, entire ecosystems could be affected. Bats are important because they help control the mosquito and crop damaging insect populations. During summer nights the average little brown bat will eat about 1,200 bugs an hour.

"Multiply that over the loss of anywhere from 5-6 million bats, it's going to have a huge impact on the agricultural industry," cautioned Yerger.

Experts estimate the loss of bats could lead to agricultural losses of $3 to $53-billion a year, and could mean an increase in the cost of crops.

"Bats are important to our environment, they're vital," said Yerger. "They're considered a keystone species which means they are essential to the way our environment works."

Scientists are still unsure how the syndrome spreads but they think it's from people entering the caves.

A count of the remaining bats at the Durham mine is planned for early next year.